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Journal of Structural Geology 34 (2012) 77e90
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Journal of Structural Geology
journal homepage: www.elsevier .com/locate/ jsg
Origin and behavior of clay minerals in the Bogd fault gouge,
Mongolia
M.D. Buatier a,*, A. Chauvet b, W. Kanitpanyacharoen c, H.R.
Wenk c, J.F. Ritz b, M. Jolivet d
aChrono-Environnement, UMR 6249, Université de Franche Comté, 16
route de Gray Besançon 25030, FrancebGéosciences Montpellier, UMR
5243, Université Montpellier 2, 34095 Montpellier Cedex 5, Francec
Earth and Planetary Science, University of California, Berkeley, CA
94720, USAdGéosciences Rennes, UMR 6118, Université Rennes 1,
Campus de Beaulieu, 35042 Rennes Cedex, France
a r t i c l e i n f o
Article history:Received 10 February 2011Received in revised
form12 September 2011Accepted 30 October 2011Available online 9
November 2011
Keywords:ClaysGougeSeismic faultTEMSEMTexture
* Corresponding author. Tel.: þ33 (0) 3 81 66 65 61E-mail
address: [email protected] (M
0191-8141/$ e see front matter � 2011 Elsevier Ltd.
Adoi:10.1016/j.jsg.2011.10.006
a b s t r a c t
We analyzed twelve fault gouge samples from the Bogd fault in
south-western Mongolia to understandthe origin and behavior of clay
minerals. The investigation relies on x-ray powder diffraction
(XRD),scanning electron microscopy (SEM), transmission electron
microscopy (TEM) and high energysynchrotron x-ray diffraction
methods to investigate microstructure and preferred orientation.
Smectite(montmorillonite), illite-smectite mixed layers,
illite-mica and kaolinite are the major clay components,in addition
to quartz and feldspars, which are present in all samples. The
observations suggest that theprotoliths and the fault rocks were
highly altered by fluids. The fluid-rock interactions allow
clayminerals to form, as well as alter feldspars to precipitate
kaolinite and montmorillonite. Thus, newlyformed clay minerals are
heterogeneously distributed in the fault zone. The decrease of
montmorillonitecomponent of some of the highly deformed samples
also suggests that dehydration processes duringdeformation were
leading to illite precipitation. Based on synchrotron x-ray
diffraction data, the degreeof preferred orientation of constituent
clay minerals is weak, with maxima for (001) ranging from 1.3 to2.6
multiples of a random distribution (m.r.d). Co-existing quartz and
feldspars have random orientationdistributions. Microstructure and
texture observations of the gouges from the foliated microscopic
zone,alternating with micrometric isotropic clay-rich area, also
indicate that the Bogd fault experienced brittleand ductile
deformation episodes. The clay minerals may contribute to a slip
weakening behavior of thefault.
� 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Fault gouge is located in a highly deformed zone resulting
fromlocalization of shear (Mair and Abe, 2008; Rutter et al.,
1986). It ischaracterized by very fine-grained materials produced
by cataclasisduring tectonic movements. Clay is generally the
dominant mineralgroup in the fault gouge but can be formed over a
wide range ofenvironmental conditions. Clay minerals also have high
cationexchange capacity and large specific surface area,
allowingadsorption of water molecules (Hofmann et al., 1933). The
attrac-tion between dipoles of water molecules and electrically
chargedclay surfaces can significantly decrease strength of fault
gouge(Bird, 1984; Morrow et al., 2000). Expandable clays in the
smectite-group, particularly the most common member such as
montmo-rillonite, can take a great amount of interlayer water into
the crystalstructure (Bird, 1984). Moreover, the preferred
orientation of
; fax: þ33 (0) 3 81 66 65 58..D. Buatier).
ll rights reserved.
constituent clays can play a crucial role on fault behaviors,
forinstance, controlling frictional and hydrological properties,
andaffecting permeability and slip rate of the faults (Haines et
al., 2009;Rice, 1992; Vrolijk and van der Pluijm, 1999). These
unique char-acteristics of clays have caught attention of
geologists as they haveimportant implications on the stability and
strength of faultmechanics. Thus, a number of experimental studies
of the micro-textural evolution and mechanical properties of
clay-bearing faultgouge have been carried out (Bird, 1984; Haines
et al., 2009;Mizoguchi et al., 2009; Morrow et al., 2000; Rutter et
al., 1986).However, the investigation of clay minerals in natural
fault gougesis very challenging due to small grain size, poor
crystallinity andvarious clay growth processes in the same
location. For example,authigenic clays can be produced by
fluid-rock interaction or directprecipitation from circulating
fluids. Furthermore, synkinematicclayminerals are produced by
deformation that commonly requiresthe presence of fluids (i.e.
Dellisanti et al., 2008; Day-Stirrat et al.,2008).
The aim of this study is to investigate microstructure
andpreferred orientation of clays at various scales in order to
have
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M.D. Buatier et al. / Journal of Structural Geology 34 (2012)
77e9078
a better understanding of their characteristics and
formationprocesses in fault gouge. We used twelve gouge samples
from theseismogenic Bogd fault in southern Mongolia (Florensov
andSolonenko, 1965; Ritz et al., 1995; Kurushin et al.,
1997).Samples were first investigated with x-ray power
diffraction(XRD) for mineralogical composition and then analyzed
withscanning electron microscopy (SEM) for microstructure.
Twoselected samples (BO25 from Bitut and BO20D from Noyon Uul)were
examined with transmission electron microscopy (TEM)techniques for
clay structures in fine detail. Synchrotron x-raydiffraction
method, which has been shown to be able tosuccessfully characterize
composition and crystallographicpreferred orientation (the term
fabric or texture is used inter-changeably) of natural and
experimentally produced clay-richsamples (Kanitpanyacharoen et al.,
in press; Voltolini et al.,2009; Wenk et al., 2008a, 2010), was
also employed on threegouge samples (BO20D, BO20E and BO24O). A
Rietveld dataanalysis (Rietveld, 1969) was applied to extract
quantitativetexture information of constituent minerals from
diffractionimages. Our texture analysis of the Bogd fault is also
comparedwith other fault systems such as the San Andreas (Janssen
et al.,2010; Schleicher et al., 2009; Wenk et al., 2010) and the
Punch-bowl faults in California (Schulz and Evans, 1998), the Moab
faultin Utah (Solum et al., 2005), the Nojima fault zone in
Japan(Shimamoto et al., 2001), and the Alpine fault in New
Zealand(Warr and Cox, 2001).
Fig. 1. Location of the studied faults in the Gobi-Altai massif
(Mongolia) (A and B)
2. Geological setting and sample selection
We selected twelve samples from the Bogd fault system insouthern
Mongolia that experienced a magnitude 8.3 Gobi-Altayearthquake in
1957 (Florensov and Solonenko, 1965; Baljinnyamet al., 1993;
Kurushin et al., 1997; Ritz et al., 1995). The Bogdfault belongs to
the Gurvan Bogd fault system that formed theeasternmost part of the
Gobi-Altay mountain range (Fig. 1). TheBogd fault corresponds to a
N100�E trending, 260 km long left-lateral strike-slip fault zone,
bounding to the North the Ih Bogdand Baga Bogd massifs, two 50 km
long, 20 km wide mountainranges situated within restraining bends
(Fig. 1). We studied theBogd and paleo-Bogd faults located at the
northern limit of the IhBogd massif. The Bogd and paleo-Bogd faults
have a sinistralreverse kinematics as illustrated at the outcrop
scale (Fig. 2). Bothfaults were re-activated during the late
Cenozoic from inheritedPaleozoic and Mesozoic structures (Florensov
and Solonenko,1965; Ritz et al., 2006; Jolivet et al., 2007;
Vassallo et al., 2007a,2007b).
Four gouge and two protolith samples were collected from theBogd
fault at the Noyan Uul location. In the following discussion,we
refer the samples as BO17, BO20B, BO13C, BO19, BO20D andBO20E. The
active Noyan Uul fault forms the base of a 40 m thickdeformation
zone characterized by multiple and alternating faultsand
cataclastic layers (Fig. 2A). The fault trends N100�E andsteeply
dips towards the south. The frontal part of the fault F0
and aerial photograph of the studied area (C) (modified after
Ritz et al., 2006).
-
Fig. 2. A - Field description of the Bogd fault at Noyon Uul,
location. The cross section represents the lithological succession
observed along the Bogd fault. Faults numbers arerelated to their
respective chronology. F0 is the younger, F2, the older. Other
illustrations show a front view of the F0 fault (sketch and
photograph). Sample locations referred to thetext are indicated. B
- Outcrop description of the PaleoBogd fault at the Bitut location
showing the gouge distribution within the different lithologies.
Two gouge generations can beseen in the figure (see text). Sample
locations are indicated.
M.D. Buatier et al. / Journal of Structural Geology 34 (2012)
77e90 79
(samples BO20D and BO20E) displays evidence of recent
seismicactivity with alluvial fan conglomerates that are part of
the frontalpeneplain domain. At this location, the fault is
characterized bya 50 cm thick gouge divided into two distinct
parts, (a) a 20 cmthick foliated gouge with sinistral shearing
(BO20E) and (b)a 20 cm thin black homogeneous gouge (BO20D). These
two partscan be clearly distinguished as shown in Fig. 2A. White
lensesrepresent cataclastically-deformed relict host-rock. We
alsosample the host rocks situated above the black gouge
level(sample BO20B).
Five additional fault gouge and one protolith samples
wereobtained from the paleo-Bogd fault at the Bitut river (Figs. 1
and2B). The paleo-Bogd is roughly 15 m thick, complex and
inactive
fault zone. A 2 cm thick black gouge forms a thin boundary
sepa-rating, gneiss in the southern part of the fault, and
meta-sedimentary quartzite and mica-schist on northern section(Fig.
2B). A 30 cm thick black and white foliated gouge is alsodeveloped
within a white quartzite unit. The shear zone contains atleast two
generations of gouge that are visible as displayed inFig. 2B and
numerous shear criteria indicates a reverse sinistralmotion. The
composition and microstructure of the fault is gener-ally
heterogeneous; thus three different types of samples werecollected.
Sample BOGD12 and BO26F are from the thick black andwhite gouge,
samples BO24O, BO24N and BO25A are from the thinblack gouge and
sample BO24H is from the quartzite protolith(Fig. 2B).
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M.D. Buatier et al. / Journal of Structural Geology 34 (2012)
77e9080
3. Experimental methods
3.1. X-ray powder diffraction (XRD)
XRD analyses of bulk sediment samples were conducted at
theGeological Institute of the University of Neuchatel with a
Sintag2000 diffractometer using Cu-Ka radiation and Germanium
crystalKevex detector. Diffraction patterns were obtained with 2q
rangefrom >0 to 60� at a scan speed of 0.5�/min. The XRD was
operatingwith an accelerating voltage of 45 kV and current of 40
mA, with0.1�e1� opening slits. Samples were prepared by grinding
roughly800 mg of sediments into fine power and pressing at about 20
MPain a powder holder covered with a blotting paper. More
detailsabout the procedure can be found in Kübler (1987).
Relativeabundances of minerals were estimated qualitatively based
on thediffraction peak heights without interferences. In order to
deter-mine the nature of clay minerals, oriented preparations of
theextracted.
-
Fig. 3. Thin section of the quartzite unit at the Bitut
location. Note the specificmineralogy with dominant quartz and mica
grains affected by carbonate veins.Qtz ¼ quartz, mus ¼ muscovite,
carb ¼ ankerite.
Fig. 4. Scanning electron microscopy (SEM) in secondary electron
imaging mode ofnewly formed ankerite crystal in the protolith
gneiss from Noyon Uul.
M.D. Buatier et al. / Journal of Structural Geology 34 (2012)
77e90 81
difficult to determine if the two samples come from the
samelithological unit of Noyan Uul. In contrast, sample from the
Bitutarea does not exhibit any trace of cataclasis (Fig. 3).
According toXRD analyses and optical microscopic observations,
quartz, feld-spars, ankerite and kaolinite are present in all
samples but indifferent proportions (Table 1). Feldspars is a major
phase in BO17,which can correspond to an altered gneiss, whereas
quartz isdominant in BO20B. BO24H is a quartz schist from the Bitut
outcropand mostly composed of mica and quartz with minor
feldspars(Fig. 3 and Table 1). XRD data and SEM images suggest that
allprotolith samples have secondary mineral phases. For
example,kaolinite replaces feldspars and newly formed ankerite
crystals areobserved as aggregates of euhedral crystals filling
secondaryporosity (Fig. 4).
According to XRD analyses on sediments with a grain size
-
Fig. 5. X-ray diffraction patterns of bulk samples of black
gouge from Noyon Uul(BO20DRT) and Bitut (BO24ORT). S: smectite, I:
illite, K: kaolinite, Qtz : quartz, F:feldspar.
Fig. 6. X-ray diffraction patterns of oriented clay fraction of
the gouge samples (afterglycolation) from Noyon Uul (BO20D <
2mmG) and Bitut (BO24O < 2mmG) SM:smectite, IL: Illite, IS:
Illite-smectite mixed layers, K : kaolinite, QTZ: quartz.
M.D. Buatier et al. / Journal of Structural Geology 34 (2012)
77e9082
4.2.1. Cataclasite zoneThe thick black gouge from Bitut has
cataclastic microstructure
with relict of the protolith mica schist and quartz grains in
varioussizes (Fig. 7A). Elongatedmicas (w100 mm) are commonly
observedand preferentially oriented, which can be related to the
relic textureof the ancient mica schist. Large quartz grains
(>300 mm) are highlyfractured with angular shape, suggesting a
fragmentation processwithout displacements (Fig. 7B, C and E). The
cataclasite zone is alsocharacterized by the occurrence of
authigenic kaolinite that fills theinterstices between fragmented
quartz grains. Small kaolinitegrains (
-
Fig. 7. SEM-BSE images of the cataclasite zone from the gouge
samples. A. oriented mica and fragmented quartz in sample BOGD12
(Bitut). B. Highly altered feldspars in sampleBOGD12 from Bitut C
fractured quartz grains and kaolinite filling porosity (sample
BO26). D. Authigenic kaolinite and small feldspar relics (sample
BOGD12). E. Highly fragmentedquartz grains (BO26). F. authigenic
kaolinite and detrital Mica (BO26). Qtz: quartz, Mi: mica, F:
feldspar, Kaol: kaolinite.
M.D. Buatier et al. / Journal of Structural Geology 34 (2012)
77e90 83
microtomed samples, small quartz grains of less than 0.5 mm
aresurrounded by very fine-grained and poorly crystallized
clayparticles, identified as CL in Fig. 10A and B. At least three
differenttypes of clays (mica Mi, kaolinite Kaol and
montmorillonite and/or
Fig. 8. SEM-BSE image at low magnification of the transition
between cataclasite and gougethe thin section preparation). Ox: Fe
oxides, Kaol: kaolinite, Qtz: quartz.
illite-smectite mixed layers (IS) are observed throughout
thesamples (Figs. 10 and 11). Micas of about 300 nm diameter are
oftenfound to be mixed with small clay particles. Clay minerals
also tendto orient themselves parallel to quartz grain boundaries
(Fig. 10D).
texture in sample BO25 (Bitut). The black area corresponds to
holes (artifact related to
-
Fig. 9. Texture of the gouges at low magnification (A, B, C and
D), and high magnification (E and F). A. Thick foliated gouge from
Bitut (sample BO26), B. Thin gouge from Bitut withintermediate
texture (sample BO25). C. Gouge from Noyon Uul with isotropic
texture (sample BO20F). D and E show the textural arrangement of
kaolinite along shear zone. F. Gougeof sample BO25 at high
magnification.
M.D. Buatier et al. / Journal of Structural Geology 34 (2012)
77e9084
Kaolinite is characterized by its strong sensitivity to electron
beamdamage. On the ion-milled samples, porosity can be observed
atgrain boundary interfaces (Fig. 10). This particular
microtextureconfirms that clayminerals have been affected by a
fracturation andbrecciation episode. Similar TEM microstructures
have beenobserved in SAFOD core samples by Janssen et al. (2011).
Fig. 11Ashows the alignment of clay particles in sample BO20D,
which helpdefine the foliation in the sample. A high-resolution TEM
imagealso reveals that these clay particles (w0.1 mm) are composed
ofregular stacking of 5e9 layers with 10 Å periodicity, suggesting
thepresence of illite (IL on Fig. 11B). Micas with a 2M polytype
are alsoidentified with a regular stacking of about 10e20 layers
andranging 0.5 to 1 micron mm in size (Fig. 11C). Irregular
stacking ofmultiple layers with 10e13 Å periodicity are also
observed insamples from the Bitut gouge (Fig. 11D). This
characteristic indi-cates the presence of smectite or
illite-smectite mixed layers (SM).
4.4. Texture measurements
All investigated samples (BO20E, BO24O and BO20D) arecomposed of
over 5 mineral phases (Fig. 12 bottom). There area large amount of
smectite (montmorillonite), kaolinite andillite-mica (fragments are
5e70 mm in size). The (001) diffractionpeaks of montmorillonite at
w14 Å are diffuse (Fig. 12), indi-cating small grain size and
considerable stacking disorder. Withthe Rietveld method (Rietveld,
1969) it was possible to modelturbostatically stacking disorder of
this particular clay phase(Lutterotti et al., 2010). Other phases
such as andesine and quartzare subordinate (5e10 wt %). Lattice
parameters of the majorphases were refined and correspond to those
described in theliteratures.
We also observed x-ray intensity changes with azimuth alongsome
Debye rings, which indicate the preferred orientation of
-
Fig. 10. TEM images of sample BO25 (ionmilled sample) showing
the textural arrangement of the gouge matrix at a micrometric
scale. A. Rounded quartz grains of 0.5 mm indiameter are surrounded
by phyllosilicates. B. Detail of the clay-quartz contact. C.
Detrital micas and kaolinite crystal with preferential orientation.
Notice that porosity is visiblebetween grain boundaries. D. Clay
smearing of quartz fragment. CL: clay (undistinguished), Mi: Mica,
Kaol: kaolinite, Qtz: quartz.
M.D. Buatier et al. / Journal of Structural Geology 34 (2012)
77e90 85
corresponding lattice planes hkl. This is best seen in a stack
ofdiffraction spectra as function of azimuth corresponding to
an“unrolled” diffraction image (Fig. 12A). Based on Rietveld
analysis(Rietveld, 1969), the refined model diffraction spectra
(top: Calc.)are compared with experimental spectra (bottom: Exp.)
in Fig. 12A,which shows a close similarity, indicative of an
excellent fit, both inintensities as well as position of
diffraction peaks. Note intensitychanges for clay minerals, but not
for quartz. The quality of the fit isfurther quantified in Fig.12B,
which displays the average spectra forthe 0� tilt image with dots
for experimental data, and a thin solidline giving the calculated
fit.
Pole figures are displayed for montmorillonite (Fig. 13A, D
andG), illite-mica (Fig. 13B, E and H) and kaolinite (Fig.13C, F
and I). Ingeneral, preferred orientation for phyllosilicate
minerals are quiteweak and asymmetric whereas orientation of quartz
and andesineare close to random (pole figures are not shown).
Kaolinite hasa stronger texture than illite-mica in all samples.
There is somevariation between samples. Kaolinite in BO20D has the
strongesttexture with a (001) maximum perpendicular to the
foliation of 2.6multiples of a random distribution (m.r.d.).
Orientation distribu-tions of montmorillonite are quite weak in all
samples, but mostrelevant in BO24O with 1.9 m.r.d. (Fig. 13D). In
all samples a-axesspin randomly around the poles to (001).
5. Discussion
5.1. Origin of clay minerals
The mineralogical investigation of the Bitut and Noyan
Uulsamples suggests that the fault gouges and protoliths are
composedof two clay types (1) the 2:1 type layers corresponding to
smectite(montmorillonite), illite and illite-smectite mixed layers,
and (2)the 1:1 type layers corresponding to kaolinite.
Montmorillonite and illite-smectite are present in
variousproportions in protolith and gouges. The BO17 sample from
NoyanUul, which is located in the damaged zone of the fault, hasa
significant amount of montmorillonite, inferring that this rock
hasundergone strong interaction with water (i.e. meteoritic or
hydro-thermal alteration). However, montmorillonite and
illite-smectiteare generally less abundant in the most deformed
zone at NoyanUul. At Bitut, the thin black gouge sample (BO24O)
from Bitutsamples also has a considerable amount of
montmorillonite. Thishas been confirmed by TEM images, which show
the occurrence ofsemi-ordered clay particles that could correspond
to authigenicmontmorillonite-like minerals. Smectite in the thin
black gouge ofBitut could be inherited from the protolith or could
be formed fromfluids in the gouges.
-
Fig. 11. TEM images of microtomed samples. A. Small crystallite
of clay minerals (illite) in the clay matrix of sample BO20D from
Noyon Uul. B. High resolution image of an illitecrystallite (newly
formed ?). C. Detrital mica with 2M polytype in sample BO20D.
Electron diffraction pattern of the crystal is showed on the left
side of the picture. D. Smectite orillite-smectite mixed layer in
sample BO25 from Bitut.
M.D. Buatier et al. / Journal of Structural Geology 34 (2012)
77e9086
Kaolinite is present in all samples and can be directly
precipi-tated from fluids. It generally occurs as filling of the
pores in cata-clasites and as deformed lenses in thin black gouges.
Theseobservations imply that kaolinite has been formed between
twodeformation events, for example, as a result of fluid
circulation inpermeable structures created by the cataclastic
deformation. Thepresence of ankerite in the protolith adjacent to
the fault is anotherevidence, suggesting that the damaged zone of
the fault wasexposed to the circulation of meteoric or hydrothermal
fluids. Thepresence of clay aggregates along the shearing planes in
the cata-clastic zone infers that they facilitate development of
shear planesand the sliding mechanism of faults.
The origin of illite in the protolith and black gouges of
thestudied outcrops is more complex. In the gouges, it is formed
asa result of fragmentation of muscovite particles that were
initiallypresent in protolith. The characterization of 2M polytype
for someof the illite particles confirmed that the particles are
inherited, i.e.their formation occurred at high temperature and
pressure and canbe related to the early metamorphism and
deformation history. Thediagenetic-metamorphic conditions can be
investigated bymeasuring the crystalline domain size of illite. An
increase of the ICwas documented by Abad et al. (2003a,b) in
phyllonite, located veryclose to a thrust plane, as a result of
reducing of crystallite size andincrease of lattice defect in
illite during deformation. We observea similar trend in our samples
and it can thus be related to faulthistory. Particularly, in Bitut
samples, the thin black gouge sampleBO24O has higher IC (or poorer
crystallinity; 0.32) than that of thehost rock sample BO24H (0.29).
This infers a strong reduction ofparticle size during deformation.
The evidence of the 2M polytypesof some illite particles from the
black gouges observed by TEM(Fig. 11) suggests that the major part
of illite is inherited fromfragmentation of metamorphic micas.
However, in the Noyan Uulsample, TEM images show that small
particles of illite are abundant
(Fig. 13). Their morphology and their 1M stacking sequences as
wellas their texture suggest a possible synkinematic origin
(Howeret al., 1963; Pevear, 1999).
5.2. Behavior of phyllosilicates
Three fault gouges (BO20E, BO24O and BO20D) which wereanalyzed
for texture have similar mineralogical composition withdominating
illite-mica (28.68e46.48 wt%). Overall, the fault gougefabrics are
weak and asymmetric, with maxima on (001) polefigures ranging from
1.27 m.r.d. in montmorillonite to 2.61 m.r.d. inkaolinite (Fig.
13). Minimum pole densities are from 0.42 m.r.d. to0.78 m.r.d.
indicating a large number of randomly oriented crystals.The texture
strength of Bogd fault gouge is consistent with theprevious
synchrotron study of the San Andreas fault gouges(1.5e2.5 m.r.d.)
(Wenk et al., 2010), as well as studies by X-raytexture goniometry
for the Punchbowl fault from California (Vander Pluijm et al.,
1994; Solum et al., 2003: 2e3.5 m.r.d.), theDeath Valley area and
West Salton detachments from California(Haines et al., 2009:
1.7e4.5 m.r.d.), the Moab fault from Utah(Solum et al., 2005: 1.8e5
m.r.d.), the Lewis thrust from Canada(Yan et al., 2001: 2e4m.r.d.),
the Caboneras fault from Spain (Solumand van der Pluijm, 2004: 2e7
m.r.d.), the Nojima fault from Japan(Shimamoto et al., 2001) and
the Alpine fault from New Zealand(Warr and Cox, 2001).
A considerable amount of stacking disordered
montmorillonite(12.43e21.68 wt%) is observed by a weak, broad and
stronglyasymmetric peak at 14 Å. This indicates interlayer water in
the claystructure, suggesting a hydrothermal alteration history.
The effectof stacking disorder is taken into account for texture
analysis byintroducing 10 layers along the stacking direction
(Lutterotti et al.,2010). There is a noticeable texture variation
between each clayphases, but kaolinite generally displays a
stronger (001) texture
-
Fig. 12. A. Diffraction images showing variation of intensity
along Debye rings. B. Map 2D plots of calculated (top) and
experimental (bottom) diffraction spectra. C. Averagediffraction
spectra showing experimental data (dotted line) and calculated
(solid line) models for BO20D sample.
M.D. Buatier et al. / Journal of Structural Geology 34 (2012)
77e90 87
(Wenk et al., 2010), as is the case in this Bogd fault gouge.
Theasymmetric pole figures of the constituent clays are referred to
theorientation of sample slab. The SEM observations suggest
thatfabrics of clays are local and heterogeneous, and thus the
foliationwas difficult to ascertain. Moreover, texture of quartz
and andesine
Fig. 13. (100) and (001) pole figures for montmorillonite,
illite-mica, and kaolinite from the Oof a random distribution
(m.r.d.).
are random. It has been shown that the presence of
water-bearingminerals in gouge can affect the frictional resistance
of the fault(Bird, 1984). Morrow et al. (2000) describes
experiments with drysheet structure minerals in presence of water
and show that theseminerals can decrease the frictional resistance
of the gouge
D of BO20E (A, B, C), BO24O (D, E, F) and BO20D (G, H, I)
samples. Contours in multiples
-
M.D. Buatier et al. / Journal of Structural Geology 34 (2012)
77e9088
material. The temperature reached during frictional heating
duringa seismic event is able to create melting of the host rock
(pseudo-tachylite) (Sibson, 1975) or to cause clay minerals
dehydration(Brantut et al., 2008; Hirono et al., 2008). However,
the productresulting from such dehydration is generally amorphous.
Brantutet al. (2008) study the frictional behavior of kaolinite and
suggestthat dehydration and amorphization of kaolinite occurs at
co-seismic slip of 1 m/s Hirono et al. (2008) investigate the
kineticsof reaction of clayminerals during frictional heating and
find that attemperatures below 200 �C, montmorillonite dehydration
ispossible in less than 105 s, whereas at temperatures higher
than800�, dehydroxylation of kaolinite and smectite can occur in a
shorttime, but illitization of montmorillonite requires much longer
time.In the Taiwan Chelungpu active fault, the decrease of
kaolinite andsmectite in the gouge is explained by the frictional
heating duringearthquake (Hirono et al., 2008). In this study, we
conclude thatfragmentation of constituent minerals is one of the
deformationmechanisms in the Bogd and paleo-Bogd gouges. This
brittleactivity of the fault allows strong fragmentation of
inheritedphyllosilicates from the host rock. For example, 2M
polytype micaswere formed in the protolith during the ductile stage
of deforma-tion (shear zone). Kaolinite is present in all samples,
suggesting thatthe studied gouges were not submitted to frictional
heatingconditions as in the experiments of Brantut et al. (2008).
Ourmicrostructure observations and texture measurements suggestthat
the Bogd and paleo-Bogd faults underwent polyphase defor-mation
processes with high fragmentation (communicationprocess) of grains
during brittle activity and fault creeping episodesalternating with
fluid circulation, which precipitated different claycomponents as a
result.
5.3. The role of clay minerals on fault weakening
Holdsworth et al. (2011) investigate the microstructures
ofsamples from the San Andreas fault drilled at 3 km depth and
finda very similar texture to that described in the present study,
withsmectite derived from fluid related processes and fault
weakeningand chemical alteration processes. In the San Andreas
fault, smec-tite forms interconnected networks of locally aligned
phyllosili-cates which can yield significantly lower friction
coefficient. Theamount of clay minerals in fault rocks can
influence the frictionalfault strength (Morrow et al., 2007; Tembe
et al., 2006). Solum et al.(2010) provide evidence for authigenesis
of clays in the Moab faultand they show that this process can lead
to a significant increase ofthe clays content on fault and
consequently induce a fault weak-ening. The relationship between
frictional strength and claymineralogy in natural fault gouge has
been investigated by Numelinet al. (2004). The friction coefficient
is measured at normal stressfrom 5 to 150 MPa. They find a
coefficient of 0.2e0.4 for samplescontaining more than 50 total
clay content, whereas for mostsamples with lower clay content the
friction coefficient is about0.6e0.7, consistent with Byerlee’s
law. Montmorillonite, which cantake on large amounts of interlayer
water, has a coefficient offriction, m of only 0.2 at room
temperature under moderate pres-sure conditions. Morrow et al.
(2000) demonstrate that the frictioncoefficient decreases more than
60% for montmorillonite in thepresence of adsorbed water.
Illitization of montmorillonite is suspected in the gouge
samplefrom Noyan Uul. The preferred orientation of the illite
particlessuggests that they formed during post seismic
creeping.Montmorillonite-illite transition is generally observed in
pelitewhich underwent progressive burial (Ahn and Peacor, 1989).
Thistransition is usually related to temperature increase and
chemicaltransfer through fluids (Buatier et al., 1992). Vrolijik
and van derPluijm, (1999) suggest that kinetic energy supplied by
fault activity
could allow montmorillonite transformation to illite in
faultgouges. Dellisanti et al. (2008) describe illitization of
smectitefacilitated by the preferential orientation of
phyllosilicates alongplanar discontinuities, which circulates the
fluids and permits thedissolution of smectite and recrystallization
of illite. Saffer andMarone (2003) compared the frictional
properties of smectite andillite rich gouges. They found a lower
friction coefficient formontmorillonite but they did not observed
any transition fromvelocity strengthening to velocity weakening
behavior duringsmectite to illite transition. Based on the results
of Lockner et al.(2006), Solum et al. (2010) speculate that the
transition frommontmorillonite to illite would cause a change from
stick-slip tocreeping fault behavior.
6. Conclusions
This study demonstrates that the protoliths and the fault
gougesof the Bogd and paleo-Bogd faults were highly altered by
fluids. Thefluid-rock interactions allow the formation of clay
minerals and thealteration of feldspars, which precipitated
kaolinite and montmo-rillonite. This alteration occurs between two
deformation episodesand affects the protolith and the fault zone.
Micas are present in allsamples and can be observed as fragmented
inherited 2M musco-vite grains in the protoliths. In some highly
deformed samples fromNoyan Uul, the presence of newly formed
synkinematic illite issuspected. The decrease of montmorillonite
fractions in highlydeformed samples from Noyan Uul implies
dehydration processduring deformation leading to illite
precipitation. Microstructuresare quite heterogeneous with foliated
microscopic zones alter-nating with isotropic clay-rich areas. The
clay textures have (001)maxima ranging from 1.27 m.r.d. to 2.61
m.r.d. Kaolinite has thestrongest texture while montmorillonite has
the weakest texturalstrength in all samples. Co-existing quartz and
andesine haverandom textures. These observations suggest that the
studied faultregistered brittle and ductile deformation although
aseismic creepalong the Bogd and paleo-Bogd faults systems was not
detectedwith the classical morphotectonic methods. The clays
minerals,which are the major component of the gouge, are mostly
related tothe circulation of fluid. Their presence may favor the
slip weak-ening behavior of the fault.
Acknowledgments
The authors thank Remi Chassagnon and Nicolas Rouge for
theirtechnical assistance on the TEM (ULB) and SEM (UFC)
analyses.HRW acknowledges support from NSF (EAR-0836402) and
DOE(DE-FG02-05ER15637). We are appreciative for access to
beamline11-ID-C at the Advanced Photon Source and assistance from
YangRen for synchrotron diffraction experiments.We are grateful for
theconstructive reviews provided by John Solum and an
anonymousreviewer.
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Origin and behavior of clay minerals in the Bogd fault gouge,
Mongolia1 Introduction2 Geological setting and sample selection3
Experimental methods3.1 X-ray powder diffraction (XRD)3.2 Scanning
electron microscopy (SEM)3.3 Transmission electron microscope
(TEM)3.4 Synchrotron x-ray texture measurement
4 Results4.1 Mineralogical characterization4.1.1 The
protoliths4.1.2 Fault gouges
4.2 Microstructural characteristics (SEM)4.2.1 Cataclasite
zone4.2.2 Clay gouges
4.3 TEM observations4.4 Texture measurements
5 Discussion5.1 Origin of clay minerals5.2 Behavior of
phyllosilicates5.3 The role of clay minerals on fault weakening
6 ConclusionsAcknowledgmentsReferences